1. Field of the Invention
The present invention relates to a contact detecting apparatus for detecting the contact of a tool with a workpiece when they are moved relative to each other.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,203,691, a conventional contact detecting apparatus is provided with a coil mounted on the end of a spindle head in which a spindle is rotatably received. This coil is connected to an AC power source to be energized, so that the coil generates loops of magnetic flux surrounding the spindle which induces current in a direction of the spindle. When a tool does not contact a workpiece, a looped secondary circuit connecting the spindle and a machine body is not completed, whereby there is no induced current. On the other hand, when the tool contacts the workpiece, the looped secondary circuit is completed to produce induced current. In this way, the secondary circuit is completed depending upon the contact between the tool and the workpiece, and an exciting current in the coil on the primary side is changed depending upon the change in electric current in the secondary circuit. Accordingly, the contact between the tool and the workpiece can be detected.
With this method, an electric resistance in the secondary circuit affects the detecting sensitivity. In particular, an oil film formed on bearings supporting the rotating spindle serves as an insulator, whereby there has to be provided a brush for bypassing an electric resistance in the bearings. This brush is easily worn out to shorten its life when the spindle is rotated at high speed. Accordingly, it is preferrable to make the brush contact at the axis of spindle rotation, where relative speed is minimum. However, in a machine tool with an automatic tool change apparatus, a clamp bar engageable with a pull stud of a tool received in a spindle passes through the spindle in coaxial relationship therewith, and an unclamping cylinder for unclamping the tool is disposed in face to face relationship with the end surface of the clamp bar, so that it was difficult to make the brush contact at the axis of spindle rotation.